1. Field of the Invention
The present invention relates to a hermetically sealed optical-semiconductor container in which an optical-semiconductor element is accommodated, and to an optical-semiconductor module using the hermetically sealed optical-semiconductor container.
2. Description of the Prior Art
A hermetically sealed optical-semiconductor container in which an optical-semiconductor element, a driver IC or the like is accommodated is used in a semiconductor device for opto-electronics which needs to operate at high speeds in the fields such as optical communication, particularly, an optical-semiconductor module such as a light exciting source or an optical-semiconductor amplifier for an optical-fiber amplifier.
In a conventional hermetically sealed optical-semiconductor container, as shown in FIGS. 1 and 2, in general, a bottom plate 2 which is made of an Fe--Ni--Co alloy, an Fe--Ni alloy such as 42 alloy, or a composite metal material such as CuW is fixed to a frame 1 which is made of a metal such as an Fe--Ni--Co alloy. e.g., Kovar. Particularly in the hermetically sealed optical-semiconductor container which consumes a large electric power and is required to have a good heat dissipation, the bottom plate 2 made of CuW is used.
The frame 1 which constitutes the side wall portion of the hermetically sealed optical-semiconductor container is fabricated by cutting or injection molding Kovar above described, and is normally provided with ceramic-terminal portions 3 formed from a plurality of ceramic sheets having metallized portions as required, and lead terminals 4 made of Kovar. There are also a structure in which part of the frame 1 is made of a ceramic, which is an insulator, and is integrated with the ceramic terminal portions 3 and a structure in which the lead terminals 4 are inserted into through-holes provided in the frame 1 and are fixed to the frame 1 by glass sealing.
Furthermore,-the frame 1 has a light transmitting window 5 through which light is transmitted between the inside and the outside of the container. The light transmitting window 5 is normally made up of a pipe made of Kovar or the like, and a window material such as glass bonded for the purpose of hermetic sealing. Some types of hermetically sealed optical-semiconductor containers use an optical-fiber transmitting window which comprises an optical fiber inserted through the frame 1 and hermetically sealed by soldering without the use of a window material such as glass. In this case, only the pipe which constitutes a window frame is joined to the frame 1 of the container.
The components such as the frame 1, the bottom plate 2 and the lead terminals 4 are joined together by silver brazing or soldering, thus assembling a hermetically sealed optical-semiconductor container. The entire hermetically sealed optical-semiconductor container is normally plated with gold for the purpose of effecting hermetic sealing with a lid at a later time, for the purpose of preventing corrosion of the container, and for the purpose of facilitating soldering during the assembly of an optical-semiconductor module. After an optical-semiconductor element and the like have been mounted inside this hermetically sealed optical-semiconductor container, a lid (not shown) is finally fixed to the top end face of the frame 1 by welding or soft soldering via a ring made of Kovar or the like.
Such a hermetically sealed optical-semiconductor container is disclosed in, for example, Japanese Patent Laid-Open No. 314747/1994. As is described in Japanese Patent Laid-Open No. 314747/1994, the bottom plate made of Kovar or CuW is in general fabricated by cutting these metals. In particular, if the bottom plate is made of CuW, its coefficient of thermal expansion differs from that of Kovar of the frame, so that the container may warp. The warp leads to the problem that the optical axis of the optical-semiconductor module fixed to a heat sink by screws may deviate. Accordingly, in the above-cited Japanese Patent Laid-Open, a flange portion of the bottom plate is made thin by cutting so that the warp is reduced and absorbed.
In Japanese Patent Laid-Open No. 82659/1994, an equivalent effect is obtained by forming only the flange portion of the bottom out of a different metal having a small coefficient of longitudinal elasticity, instead of thinning the flange portion. However, it is extremely difficult to join different kinds of metals at joining portions having small areas and obtain a sufficient strength.
An optical-semiconductor module, as shown in FIG. 3, includes a circuit board 7 on which are mounted an optical-semiconductor element such as a laser diode (LD) element 6 or a photodiode (PD) element, a driver IC for driving the optical-semiconductor element, a chip thermistor for temperature measurement and the like, and the above-described type of hermetically sealed optical-semiconductor container in which the circuit board 7 is mounted. However, the LD element tends to vary in oscillation wavelength with temperature, and also has the disadvantage that its optical output lowers or its life becomes extremely short under high-temperature conditions, leading to a decrease in reliability.
For this reason, an electronic cooling device is used for controlling the temperature and cooling the LD element and the like. This electronic cooling device has a structure in which, as shown in FIG. 3, a plurality of electronic cooling elements (Peltier elements) 8 each of which is made of a crystal or a sintered body of BiTe which is a compound semiconductor are sandwiched between two insulator substrates 9 each of which comprises a ceramic plate and on which electrodes and interconnections are provided by metallization. Alumina or aluminum nitride is in general used for the insulator substrates 9 of the electronic cooling device. In particular, in cases which high heat dissipation capability is needed or reduce the power consumption of the electronic cooling device is reduced, aluminum nitride (AlN) having a good heat conductivity is used for the insulator substrates 9.
Each of the electronic cooling elements 8 has a characteristic degradation of the electronic cooling elements degrade the cooling efficiency of the electronic cooling device, increasing the power consumption thereof. In the worst case, it leads to the problem that the temperature control of the optical-semiconductor module is impossible owing to its self-heating.
This problem is caused by the fact that temperature variation causes warp of components such as the bottom plate of the hermetically sealed optical-semiconductor container and the circuit board on which the LD element or the PD element in the optical-semiconductor module are mounted, and thermal stress is concentrated in the electronic cooling elements such as BiTe elements which have a low Young's modulus and are comparatively soft, with the result that cracks occur in the electronic cooling elements.
It is considered that the warp (stress strain) of the bottom plate of the hermetically sealed optical-semiconductor container, which causes the above-described problems, occurs due to the following major reasons, i.e., (1) thermal stress strain of the bottom plate due to the difference in coefficient of thermal expansion between the frame of the container and the bottom plate, and (2) thermal stress strain of the bottom plate due to the difference in coefficient of thermal expansion between the bottom plate of the container and the insulator substrates of the electronic cooling device. It is also considered that there are other reasons; for example, (3) the bottom plate is warped due to thermal stress strain due to the difference in coefficient of thermal expansion between the insulator substrates of pair of leads which are electrically joined by the interconnection formed by metallization on the insulator substrates 9 for electrical connection with the hermetically sealed optical-semiconductor container. Incidentally, in the assembly of the optical-semiconductor module, after this electronic cooling device has been soldered to the bottom plate 2 of the hermetically sealed optical-semiconductor container, the circuit board 7 on which the optical-semiconductor element such as the LD element 6 or the PD element and other components are mounted is fixed to either one of the insulator substrates 9 of the electronic cooling device by soldering.
Incidentally, Japanese Patent Laid-Open No. 67844/1993 has proposed an optical-semiconductor module in which the bottom plate of a hermetically sealed optical-semiconductor container is used as the insulator substrate of an electronic cooling device, i.e., a method of forming the bottom plate of the hermetically sealed optical-semiconductor container out of a ceramic such as AlN. In this method, since one of the insulator substrates of the electronic cooling device can be omitted, it is possible to realize semiconductor modules having smaller sizes, particularly, thinner shapes.
After the hermetically sealed optical-semiconductor container which includes portions made of different kinds of materials in the above-described manner has been assembled into an optical-semiconductor module, if the optical-semiconductor module is subjected to an environment resistance test of -40.degree. C. to +125.degree. C. which is specified in MIL-STD, the characteristics of electronic cooling elements such as BiTe elements occasionally degrade. The the electronic cooling device and the circuit board, and (4) a heat sink to which the bottom plate is fixed is occasionally warped by the entire strain and the warp of the heat sink causes the bottom plate to warp.
Further, there is a case in which the warp of the bottom plate of the hermetically sealed optical-semiconductor container may not cause defects in the electronic cooling device but may deviate the optical axis of an optical system which provides optical coupling between an optical-semiconductor element such an LD element or a PD element and an optical fiber. This deviation of the optical axis leads to the problem of lowering the output at the optical fiber end of the optical-semiconductor module.
The method of forming the bottom plate of the hermetically sealed optical-semiconductor container from a ceramic such as AlN, as described in the above-cited Japanese Patent Laid-Open No. 67844/1993, has the-disadvantage that screwing hole portions or the like are difficult to form in the bottom plate and a portion which surround the screwing hole portions of the bottom plate is easily broken during screwing. In addition, the method has other problems; for example, even after the optical-semiconductor module and the heat sink have been screwed together, the bottom plate is easily broken when the heat sink warps.